Constraints on the origin of slab and mantle wedge anomalies in Tonga fromthe ratio of S to P velocities

We examine two prominent upper mantle Velocity anomalies in the southwest P acific, the Tongs slab anomaly and the corresponding overlying mantle wedge anomaly, using data collected during a combined land-sea deployment of tem porary seismometers. The linear geometry and small interstation spacing of the instruments yield high-resolution data along a cross section of the Ton ga subduction zone, including the actively spreading Lau back are basin. We estimate the relative variation of P and S velocity, often described as v = delta lnV(s)/delta lnV(p), for the slab and mantle wedge anomalies using two distinct methods: a linear regression of the P and S travel time residu als, and detailed modeling of the velocity structure using a three-dimensio nal finite difference travel time algorithm. The two methods yield similar results, with v of the slab being 1.1-1.5 and v of the mantle wedge being 1 .2-1.3. These values are consistent with experimental data concerning the e ffect of temperature on P and S wave velocities in the upper mantle and are lower than what is expected for velocity anomalies generated by the presen ce of partial melt. These observations imply that either the theoretical es timates of v for partial melt are too large or very little partial melt is present beneath the Lau basin. In the latter case, melt must be quickly rem oved from the rock matrix, such that the velocity anomalies are due to incr eased temperature, and not melt. The bulk of the velocity anomaly in the ma ntle wedge can be explained by temperature anomalies of 400-600 degrees C b ecause of the amplification of temperature derivatives of seismic velocity by anelastic effects. Such large thermal anomalies, generated by decreased lithospheric thickness and mantle upwelling beneath the fast spreading Lau back are basin, can still leave the mantle near the solidus, even after acc ounting for the effect of increased volatile content in the mantle wedge. T he lower-amplitude velocity reductions in the deeper wedge are likely relat ed to an increased concentration of volatiles from the subducting slab.